The present invention relates generally to a vehicle transmission of the mechanical type which includes an automatic shifting mechanism and, in particular, to a mechanical transmission wherein the lower gear ratios are manually selected by the vehicle driver, and wherein at least one of the upper gear ratios is automatically selected.
The majority of vehicle transmissions can generally be divided into two groups, "mechanical" transmissions and "automatic" transmissions. In its most common usage, an automatic transmission typically refers to a transmission which uses a torque converter and one or more planetary gear sets to achieve various predetermined output gear ratios. The automatic transmission is shifted by means of a plurality of hydraulically actuated clutch packs which are selectively engaged to lock components of the planetary gear sets relative to one another or to the transmission housing in order to achieve the desired gear ratio. On the other hand, a mechanical transmission, also referred to as a manual transmission, typically includes a gear train providing a plurality of various gear ratios which are manually selected by the vehicle driver, typically by disengaging an associated clutch, and then moving a gear shift lever to the desired gear position to cause the transmission to shift to the desired gear ratio.
While automatic transmissions offer advantages over mechanical transmissions in terms of convenience and less driver fatigue due to shifting, mechanical transmissions typically provide advantages over automatics with regard to cost, torque capacity, and fuel economy. As a result, the majority of medium and heavy duty truck transmissions are of the mechanical type. Mechanical transmissions can be further classified as either a single stage transmission, wherein all of the gear ratios available from the transmission are located within a single housing and are accessible by the driver via a single gear shift lever, or a compound transmission, wherein a single stage transmission functions as the main section of the transmission, and an auxiliary section is connected thereto to provide a plurality of additional ratios which, when combined with the ratios provided by the main section, enables the transmission to provide a larger number of gear ratios.
In order to improve the convenience of mechanical transmissions, various approaches have been proposed for automating or semi-automating the shifting of a conventional mechanical transmission. In these automated mechanical transmissions, the conventional driver operated shift tower is typically replaced by a hydraulically or pneumatically controlled automatic shifting mechanism, and an actuator is provided to automatically control the operation of the clutch.
It is also been proposed to provide a mechanical transmission wherein only a portion of the gear ratios are automatically selected. For example, the lower gear ratios can be manually selected by the vehicle driver, and the higher gear ratios can be automatically selected. Such an approach is advantageous in that the transmission is lower in cost than a fully automated mechanical transmission, yet offers automatic shifting in the upper highway range where the majority of shifting normally occurs. Thus, fuel economy is improved and driver fatigue is reduced.
U.S. Pat. No. 4,722,248 discloses a compound transmission wherein only a portion of the gear ratios are automatically selected. In particular, in this patent, all of the gears of the main section of the transmission must be manually selected by the vehicle driver but, once the driver has manually shifted the main section into its highest ratio, sequentially related higher gear ratios provided by an auxiliary section are automatically shifted.